The UK Border Agency is detaining children in "degrading and disgraceful" conditions at Heathrow, according to an official watchdog.

The Heathrow independent monitoring board (IMB) says children of all ages are being detained at the airport for immigration purposes almost every day, and are sometimes kept overnight. They are held in rooms that are small, stuffy and have no natural light. There is no access to the open air, no sleeping accommodation and only hand basins for washing. They often share space with unrelated adults and can be held in these conditions for many hours.

The annual report of the watchdog, which visits and monitors UKBA's short-term holding facilities at Heathrow, says the units at each terminal are no more than waiting rooms with rows of seats and little else. There are separate rooms for children, with cots, toys and children's books, but these are very small. The worst at terminal 3 is just nine square metres, with the result that children use the main rooms.

The IMB says suitable accommodation for families with children needs to be provided. "Until this can be done, UKBA should make arrangements for any families held at the airport to be detained at a nearby hotel or other suitable space and for transport to be available promptly," it says.

Nearly 15,000 people were detained by immigration officials at Heathrow for short periods in 2011, with nearly 3,000 of them held for more than 12 hours. Separate figures for children are not given, but one child is recorded as having spent 31 hours and 50 minutes detained at Heathrow before a social worker arrived on 20 December 2011.

"The conditions under which children are being held, and detainees have to endure overnight, are degrading and disgraceful," says the IMB's annual report. "The detention of children continues, despite the government saying that it would end the detention of children for immigration purposes. The IMB recognises that children cannot always be admitted to the country straight away and are sometimes held for their own protection. It recommends that non-custodial, child-friendly accommodation is provided at Heathrow for families with children as a matter of urgency." It adds that the UKBA has failed to deliver improvements to the accommodation, despite promises in response to previous IMB reports.

The private security company Reliance took over running the Heathrow holding rooms in May last year. The IMB says detainees have been well cared for by Reliance staff, but transport to and from Heathrow is not well organised and detainees face long waits to be taken from the airport to immigration detention centres.

A border force spokesperson said: "We share the independent monitoring board's concerns about the quality of the accommodation provided by BAA [the airport operator]. We have raised this with them on numerous occasions in the past and will continue to do so to ensure those held at the border have proper facilities to meet their and our needs.

"The report rightly recognises we are handling cases efficiently and professionally and that it identifies improvements in the way passengers, including children, are treated by staff. We will respond to the report fully in due course."

The rooms at Heathrow were built to UKBA specifications, the IMB says.